The present invention relates to a method of fabricating a part made of composite material of oxide/oxide type, i.e. comprising fiber reinforcement made of refractory oxide fibers that is densified by a matrix likewise made of refractory oxide.
The great majority of oxide/oxide composite material parts are prepared in the following two manners:
making a fiber texture by stacking two-dimensional plies of woven oxide fiber fabric and impregnating the texture with a suspension containing oxide fillers, the filled preform then being subjected to sintering; or
making a fiber texture by winding oxide fiber yarns that have previously been immersed in a suspension containing oxide fillers, the filled preform then being subjected to sintering.
Nevertheless, the mechanical characteristics of oxide/oxide composite materials obtained by those methods of preparation remain limited in certain directions. In particular, those materials present low shear strength.
Making fiber textures obtained by three-dimensional weaving between continuous warp and weft yarns enables the mechanical strength of the material to be increased. Under such circumstances, it is possible to cause a filled suspension to penetrate into the fiber texture, which may be of thickness that reaches several tens of millimeters, depending on the intended application only by methods that make use of a pressure gradient, such as injection molding type methods known as resin transfer molding (RTM), or else submicrometer powder suction (SPS).
The volume fraction of fibers in oxide/oxide composite materials is one of the fundamental parameters to be taken into consideration for achieving the final mechanical properties of the material. That is why, the fiber texture needs to be compacted by using specific tooling while preparing an oxide/oxide composite material. During the step of drying in tooling, a network of cracks forms in the matrix present between the yarns. The network of cracks leads to the impregnated preform expanding during unmolding, thereby leading to a great reduction in the mechanical properties of the material in the final stage.
One known way of remedying that expansion consists in adding an organic binder, e.g. polyvinyl acetate (PVA), to the initial suspension, thereby contributing to the cohesion of the system after the preform has dried by preventing the above-described expansion phenomenon.
In order to obtain high mechanical characteristics in a single impregnation step, it is necessary to use a slip having a sufficient volume percentage of fillers. Under such circumstances, adding an organic binder modifies the behavior of the slip. Its behavior changes from being Newtonian behavior (viscosity independent of the flow speed) to a shear-thinning behavior (viscosity dependent on the flow speed). This phenomenon leads to the appearance of nonuniformity in the composition within the material when using preparation by means of the SPS technique.
More precisely, considering the material as a whole, poor control over the flow of the suspension inside the fiber texture leads to the appearance of a composition gradient (fiber/matrix ratio) across the thickness of the material. More locally within the material, pores of uncontrolled size and distribution form once the fillers have been stabilized by sintering.